Between 1985 and x there was a period of no gains in the efficient of passenger vehicles (automobiles). Regulations required efficiency gains to reduce fuel consumption to reduce emissions and reduce reliance on imported crude oil.

2009

Explanation:

During the 1990s and early 2000s, the overall sale of more efficient cars decreased due to more strict fuel efficiency regulations on trucks and SUVs that actually increased the total number off trucks and SUVs sold, decreasing the average fuel efficiency by 1 mile per gallon.

It wasn't until President Obama passed stricter fuel efficiency regulations for all types of passenger vehicles that the overall efficiency increased.

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